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For Buildings of Unusually Tight Construction
1) Openings must be installed between the boiler room and the outdoors or a ventilated space, such as an attic or crawl
space, which communicates directly with the outdoors.
2) Two openings are required. The top edge of the upper opening must be within 12 inches of the ceiling. The bottom
edge of the lower opening must be within 12 inches of the floor.
3) Size openings and ducts as follows:
• Vertical ducts or openings directly outdoors (Figure 4, Figure 5, and Figure 6) - Each opening must have a free
cross sectional area of 1 square inch per 4000 BTU/hr of the total input of all gas-fired appliances in the boiler
room but not less than 100 square inches. Minimum opening size is 3 inches.
• Openings to outdoors via horizontal ducts (Figure 7) - Each opening must have a free cross sectional area of 1
square inch per 2000 BTU/hr of the total input of all gas fired appliances in the boiler room but not less than 100
square inches. Minimum opening size is 3 inches.
• The “free area” of an opening takes into account the blocking effect of mesh, grills, and louvers. Where screens
are used, they must be no finer than ¼” (4 x 4) mesh.
FIGURE 4: ALL AIR FROM OUTDOORS,
VENTILATED CRAWL SPACE AND ATTIC
FIGURE 5: ALL AIR FROM OUTDOORS,
VIA VENTILATED ATTIC
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